I did a quick search before posting this message and noticed that a few people already posted about this issue, and that the near consensus has been that it is resolved by doing a complete settings reset, which has also been my experience. This happens to me whenever I restore from iCloud, whether on the same phone or on a new phone entirely. It has persisted across devices, and I think it started with either iOS 14 or 15. Has anyone found a different solution?
For those who may not be familiar with the issue, if you turn on Speak AutoText under Accessibility / Spoken Text / Typing Feedback in order to hear autocorrect read out to you while you type on the touch keyboard, it will ultimately randomly turn itself off after a while for no apparent reason. I suspect many people, especially Bluetooth or braille display users, do not enable this setting (it is disabled by default), as it may more chatter than necessary, but it is a life saver for touch keyboard users.